def test_validate_dict_or_none(self):
        dictionary, constraints = self._construct_dict_and_constraints()

        # Check whether None is a valid value.
        msg = attributes._validate_dict_or_none(None, constraints)
        self.assertIsNone(msg, 'Validation of a None dictionary failed.')

        # Check validation of a regular dictionary.
        msg = attributes._validate_dict_or_none(dictionary, constraints)
        self.assertIsNone(msg, 'Validation of a valid dictionary failed.')

    def test_validate_dict_or_empty(self):
        dictionary, constraints = self._construct_dict_and_constraints()

        # Check whether an empty dictionary is valid.
        msg = attributes._validate_dict_or_empty({}, constraints)
        self.assertIsNone(msg, 'Validation of a None dictionary failed.')

        # Check validation of a regular dictionary.
        msg = attributes._validate_dict_or_none(dictionary, constraints)
        self.assertIsNone(msg, 'Validation of a valid dictionary failed.')
        self.assertIsNone(msg, 'Validation of a valid dictionary failed.')